First stanc3 release candidate!

Here we have 80 models working from the stat_comp_benchmarks and example_models repos, as well as the "mother model" that @mitzimorris designed to stress test the compiler.

Killer features we currently don't have hooked up that we hope to by the official 1.0 release:

• The optimizer - currently it has a bunch of standard compiler passes and quite a few symbolic replacements that help with performance and numerical stability, but it's not tested fully yet.
• Model class additions
• Faster compile times

To try out this compiler with CmdStan, simply download the binary for your operating system from the release binaries below and drop it into the cmdstan/bin directory renamed as stanc (so e.g. mv ~/Downloads/linux-stanc cmdstan/bin/stanc) - everything else should just work from then on (assuming you don't modify any of the compiler C++ files and cause CmdStan to regenerate bin/stanc from source).